1. Field of the Invention
The present invention relates to the formation of a frozen layer of fluid on bearing and seal surfaces and more particularly on thrust and support bearings and sealing rings preferably though not exclusively suited for use in easily lubricated environments such as for vertical and horizontal shaft turbines, pumps, impellers and the like.
2. Description of the Prior Art
Devices for frictionally supporting two relatively movable members are obviously well known. Amongst such devices thrust bearings and support bearings occupy an important position.
Thrust and support bearings for light equipment have reached a state of satisfactory perfection. However, the same cannot be said about bearings for heavy equipment, such as the thrust bearings for a 1000 Ton turbine rotor.
Known thrust and support bearings comprise an arrangement of pads or segments respectively, pivotally mounted generally about an eccentric or concentric axis and positioned around the shaft of the device to be supported, such shaft being integral with a ring or spider defining a glass polished friction surface arranged to contact and slide over the also smooth and flat surfaces defined on the pads or segments. Such bearings although adequate, leave room for improvement. However, the difficulty in arriving at efficient structures for thrust bearings resides in the delicate mechanical mounting (pivots, plural disc support structures, etc.) and adjustment of parts required so that all the pads support the same load, for which they must be maintained in a perfectly flat horizontal plane in spite of loads and thermal gradients. In the case of support bearings the arcuate segments also require precise mounting techniques which have led research towards other structures.
In the case of the pads of a thrust bearing or the arcuate segments of a support bearing, the running surfaces are usually of thick mild-steel plate thoroughly annealed, tinned and lined with hard (tin base) white metal. Utmost care is necessary to secure adhesion at all points to which effect electrical conductivity tests must be performed. Thickness of the white metal must be restricted to 0.05 ins, to avoid thermal distortion. The surfaces are finish machined after metalling.
These known arrangements have not only the just mentioned drawbacks but also those tied to the necessity of providing:
A. Costly lubricating systems for maintaining a lubricant permanently between the surfaces in frictional contact;
B. The lubricant itself;
C. Safety devices such as fire extinguishing equipment due to the possibility of combustion of the lubricants;
D. Periodical stoppage to service unevenly worn or thermally distortioned pads and segments; and
E. Complicated pivoted supporting structures including hydraulic pistons or jack screws for equalizing the load on each pad or segment inasmuch as uneven loading causes not only uneven wear between different pads but also deflection of the pads themselves.
f. Discs between the pads and the pivots to cause a more even distribution of the load and reduce pad distortion;
g. Radially elongated pads which due to their radial dimension make it impossible to attain sufficiently flat surfaces.
The search for satisfactory bearing arrangements has been brought about by the continued increase in the demand for electric power, whereby hydroelectric plants have flourished and for economy the corresponding generators have had to grow to sizes far above even the most optimistic foresight of our predecessors. In this respect, the necessity of producing more power by means of a single unit has tended to a reduction of the rotational speed of turbines, thereby considerably increasing their physical size, as well as that of the rotating part of the generator. By the way of example, it is estimated that by 1980, 1,000,000 HP Francis units will be in production with runners of 15 m diameter and 6 m height. The corresponding generator will have an even larger diameter and weight twice as much whereby the combination of both will require a 3,000 Ton supporting thrust bearing so as to operate satisfactory.
There is an additional problem to be considered in the art of designing turbine components and it relates to the structure of the shaft. To date the turbine and the rotor of the generator have always been mounted on a costly and heavy shaft of forged steel and supported on one or more thrust bearings comprising oscillating pads annularly mounted about the shaft. In such arrangements the torsional forces are transmitted from the periphery to the centre of the runner of the turbine, along the shaft and then from the centre of the rotor of the genrator to the periphery. If an adequate thrust bearing arrangement were provided, it would be possible to replace the forged steel shaft by a tubular cone shaped shaft whereby the transmission of the torsional forces would be from the periphery of the runner directly to the periphery of the rotor of the generator, thereby reducing the weight of the arrangement, the manufacturing costs of the shaft, etc, particularly taking in consideration the limitations of available forging equipments. It is obvious that a shaft of laminated steel is of easier construction and requires less sophisticated technology than the construction of a delicately balanced solid shaft.
The provision of a hollow shaft affords the following advantages in addition to those already mentioned:
a. Less overall weight of the rotating parts, thereby reducing costs.
b. Air suctioning through the shaft so as to avoid caving-in at the turbine. Hydraulic stability ensures less stress on the turbine runner and higher efficiency.
c. Demounting of the runner through the rotor of the generator without demounting the latter.
d. A more compact turbine and generator design leading to a less deep perforation in the ground on which the hydroelectric plant is built, a less voluminous armoured cement structure and a lower overhead crane arrangement.
However, a hollow shaft of larger diameter than a solid steel shaft requires that the thrust bearing be removed from its conventional position around the shaft and placed near the centre line of the rotor of the generator.
This solution has been held so far to be impractical and only becomes feasible with the bearing arrangement of the present invention.
Although reference has been made so far to bearing arrangements, it is also of importance to note that sealing devices, such as sealing rings for sealing-off one axial portion of a space between two relatively movable cylindrical nested members, from another axial portion of that space, are also well known in the art, and although they serve their function satisfactorily, it is known that in order to obtain precisely coinciding frictional sealing contact between one of said members and the sealing device, it is necessary to provide extreme manufacturing accuracy.